Lithium-ion batteries (LIBs) have attracted extensive attention in the past two decades for a wide range of applications in portable electronic devices such as cellular phones and laptop computers. Due to rapid market development of electric vehicles (EV) and grid energy storage, high-performance, low-cost LIBs are currently offering one of the most promising options for large-scale energy storage devices.
Currently, electrodes are prepared by dispersing fine powders of an active battery electrode material, a conductive agent, and a binder material in an appropriate solvent. The dispersion can be coated onto a current collector such as a copper or aluminum metal foil, and then dried at an elevated temperature to remove the solvent. Sheets of the cathode and anode are subsequently stacked or rolled with the separator separating the cathode and anode to form a battery.
The lithium-ion battery manufacturing process is sensitive to moisture. A battery with high water content leads to serious attenuation of electrochemical performance and affects stability of battery. Therefore, environmental humidity must be controlled strictly for the production process of LIBs. Most of the LIBs are produced in an environment with less than 1 percent humidity. However, significant cost is incurred because of the stringent moisture-free process. To address the moisture sensitive issue of electrode assembly, it is important to dry the electrode assembly prior to electrolyte filing so as to reduce the water content in the battery.
Chinese Patent No. 104142045 B describes a method of drying an electrode assembly of LIBs. The method comprises heating an electrode assembly under vacuum at a temperature of 30-100° C.; filling the oven with dry air or inert gas; repeating these two steps for 1-10 times. This method provides the electrode assembly with a water content between 430.5 ppm and 488.1 ppm.
Chinese Patent Application No. 105115250 A describes a method of drying an electrode assembly of LIBs. The method comprises heating an electrode assembly under vacuum at a temperature of 85±5° C.; filling the oven with hot, dry nitrogen gas; repeating these two steps for 10-20 times. This method provides the electrode assembly with a water content of less than 200 ppm.
None of the above patent references discloses any binder composition in the electrodes for evaluating the relationship between the drying profile and binder composition. In addition, the water contents of the electrode assemblies as dried by the existing methods range from a hundred ppm to several hundreds ppm, which may affect the cycling stability and rate capability of LIBs. Even if a battery is manufactured using the electrode obtained by the above method, exfoliation of an electrode layer may occur and sufficient durability of the electrode layer cannot be obtained.
JP Patent No. 5523678 B2 describes a positive electrode for a nonaqueous electrolyte secondary battery, having a current collector and an electrode layer containing an active material, a conductive agent and a binder, wherein the electrode layer has a percentage of void of 33.0% or more to retain sufficient amount of electrolyte. However, the output performance will be affected due to decreased energy density of a battery. In addition, the peeling strength between the electrode layer and the current collector is determined by the surface roughness of the current collector.
JP Patent No. 4984384 B2 describes a method for preparing an electrode, having a current collector and an active material layer containing an iron-containing active material and a binder. However, the peeling strength between the active material layer and the current collector is also determined by the surface roughness of the current collector.
In view of the above, there is always a need to provide a nonaqueous electrolyte rechargeable battery using electrodes having high durability by inhibiting exfoliation of an electrode layer and good electrochemical performance.